Continuous treatment of a polyacrylonitrile at elevated temperatures on a porous roll

ABSTRACT

AN IMPROVED CONTINUOUS PROCESS AND APPARATUS ARE PROVIDED FOR THE UNIFORM STABILIZATION OF A STRAND OF POLYMERIC FIBROUS MATERIAL WHICH IS CAPABLE OF UNDERGOING THERMALSTABILIZATION. THE STRAND IS CONTINUOUSLY WOUND IN A PLURALITY OF TURNS AND CONTINUOUSLY UNWOUND FROM AT LEAST ONE ROTATING ROLL HAVING A POROUS SURFACE WHILE A GAS AT AN ELEVATED TEMPERATURE IS EXPELLED OUTWARDLY THROUGH THE SURFACE OF THE POROUS ROLL AND PENETATES THE FIBROUS CONFIGURATED OF THE STRAND WOUND UPON THE ROLL. IN A PREFERRED APPARATUS IN ACCORDANCE WITH THE PRESENT INVENTION THE POROUS ROLL SITUATED WITHIN A HEAT TREATMENT CHAMBER IS INTERNALLY PROVIDED WITH A PLURALITY OF INDIVIDUALLY ADJUSTABLE HEATING ELEMENTS ALONG ITS LENGTH. THE RESULTING STABILIZED MATERIAL RETAINS ITS ORIGINAL FIBROUS CONFIGURATION ESSENTIALLY INTACT, EXHABITS ENHANCED THERMAL STABILITY,AND IS CAPABLE OF UNDERGOING CARBONIZATION. IN A PARTICULARLY PREFERRED EMBOIDIMENT OF THE INVENTION THE PRECURSOR IS AN ACRYLONITRILE HOMOPOLYMER AND AIR HAVING A TEMPERATURE OF AT LEAST ABOUT 260* C. IS EXPELLED THROUGH THE SURFACE OF THE ROTATING POROUS ROLL.

y 0, 1914 D. E. STUETZ 3,826,611 CONTINUOUS TREATMENT OF A POLYACRYLONITRILE AT ELEVATED TEMPERATURES ON A POROUS ROLL Original Filed Oct. 10, 1969 2 Sheets-Sheet 1 FIG. 1

y 0, 1914 D. E. 5mm 3,826,611

CONTINUOUS TREATMENT OF A POLYACRYLONITRILE AT ELEVATED TEMPERATURES ON A POROUS ROLL Original Filed Oct. 10, 1969 2 Sheets-Sheet 2 'United' States Patent Office 3,825,611 Patented July 30, 1974 U.S. Cl. 8-115.5 Claims ABSTRACT OF THE DISCLOSURE An improved continuous process and apparatus are provided for the uniform stabilization of a strand of polymeric fibrous material which is capable of undergoing thermal stabilization. The strand is continuously wound in a plurality of turns and continuously unwound from at least one rotating roll having a porous surface while a gas at an elevated temperature is expelled outwardly through the surface of the porous roll and penetrates the fibrous configuration of the strand wound upon the roll. In a preferred apparatus in accordance with the present invention the porous roll situated within a heat treatment chamber is internally provided with a plurality of individually adjustable heating elements along its length. The resulting stabilized material retains its original fibrous configuration essentially intact, exhibits enhanced thermal stability, and is capable of undergoing carbonization. In a particularly preferred embodiment of the invention the precursor is an acrylonitrile homopolymer and air having a temperature of at least about 260 C. is expelled through the surface of the rotating porous roll.

This is a division of application Ser. No. 865,334, filed Oct. 10, 1969 now Pat. No. 3,632,092.

BACKGROUND OF THE INVENTION In the past procedures have been proposed for the conversion of organic polymeric fibers to a modified form possessing enhanced thermal stability. Such modification has generally been accomplished by heating the fibrous material in an appropriate atmosphere at moderate temperatures for extended periods of time. The product may be suitable for use as an intermediate in the formation of carbonized fibrous materials, or for direct utilization in applications where fibers of enhanced thermal stability are required.

U.S. Pat. Nos. 2,913,802 to Barnett and 3,285,696 to Tsunoda disclose representative processes for the conversion of fibers of acrylonitrile homopolymers or copolymers to a heat resistant form wherein the fibers are heated in an oxygen-containing atmosphere. Such prior art stabilization techniques have commonly been directed to batch operations which require relatively long heating periods.

The more recent Belgian Pat. No. 700,655 discloses a procedure whereby a continuous length of an acrylonitrile copolymer may be continuously subjected to a stabilization treatment to produce essentially complete oxygen saturation while maintained in air at a temperature not exceeding 250 C., e.g. three hours or more at 220 C. Commonly assigned U.S. Ser. No. 749,957, filed Aug. 5, 1968 of Dagobert E. Stuetz additionally discloses an improved generically defined process wherein certain acrylic fibrous precursors surprisingly may be continuously stabilized at more highly elevated temperatures than previously deemed possible.

Heretofore a common technique for the continuous stabilization of a continuous length of fibrous material has involved the passage of a length of the material through the interior of a tubular mufile furnace provided with an appropriate atmosphere. Rollers have commonly been situated at each end of the furnace to direct movement of the strand so that it is aligned along the axial center of the furnace. The residence time achieved within such a heat treatment zone is by necessity determined by the length of the furnace and by the strand speed. In order to achieve a commercially acceptable production level the oven (1) must be extremely long in order to make possible a relatively high strand speed, or (2) the strand must be returned for a multiplicity of passes. When using such an arrangement, the stringing of the apparatus prior to carrying out the process is often tedious and time consuming.

In continuous preoxidation systems of the prior art in which the strand being treated is wound upon a pair of spaced converging rolls and heated air is supplied to the same from an external source, it has been essential to provide a plurality of ovens in series if the strand is to be subjected to a plurality of successively elevated temperatures. Also, an appreciable time lapse may be required for the oven to attain the operating temperature desired.

When stabilization of a strand of polymeric fibrous material is conducted in accordance with the teachings of U.S. Ser. No. 865,333 filed Oct. 10, 1969 (now abandoned) in the name of William M. Cooper, by contact with the surface of a rotating heated roll having an internal heat source and a gas impervious surface, the stabilization reaction has a tendency to be at least initially concentrated upon the side of the fibrous strand where direct physical contact is made with the heated roll.

It is an object of the invention to provide an efficient continuous process and apparatus for the stabilization of a strand of polymeric fibrous material in which the strand undergoes stabilization on a highly uniform basis.

It is an object of the invention to provide an improved stabilization apparatus capable of accomplishing the continuous stabilization of a strand of a polymeric fibrous material while accommodating a relatively large precursor inventory per unit of area.

It is an object of the invention to provide an efficient process for the preoxidation of an acrylic yarn to render the same capable of undergoing carbonization.

It is another object of the invention to provide a compact stabilization apparatus capable of penetrating a fibrous strand with a hot gas at successively elevated temperatures as the strand travels along the surface of a porous roll.-

These and other objects, as well as the scope, nature, and utilization of the invention will be apparent from the following detailed description and appended claims.

SUMMARY OF THE INVENTION It has been found that a process for the stabilization of a strand of polymeric fibrous material capable of undergoing thermal stabilization comprises passing the strand through a heat treatment zone wherein the strand is con-, tinuously wound in a plurality of turns and continuously unwound from at least one rotating roll having a porous surface while a gas at an elevated temperature is expelled outwardly through the surface of the porous roll and penetrates the fibrous configuration of the,strand wound upon the roll, and continuously withdrawing the resulting stabilized strand of fibrous material from the heat treatment zone which retains its original fibrous configuration essentially intact, exhibits enhanced thermal stability, and is capable of undergoing carbonization. I

An apparatus for the continuous stabilization of a strand of polymeric fibrous material comprises a substantially enclosed heat treatment chamber, means for continuously introducing a strand of fibrous material into the heat treatment chamber, means for continuously withdrawing a strand of fibrous material from the heat treatment chamber, at least one rotatable roll situated within the chamber having a porous surface, drive means capable of rotating the roll, conduit means essentially coextensive with the length of the roll in a communicative relationship to the porous surface of the roll, heating means situated within the rotatable roll, and means for supplying a gas to the conduit means.

DESCRIPTION OF THE DRAWINGS FIG. 1 is a perspective view partially cut away of a preferred apparatus of the present invention showing the positioning of a strand of polymeric fibrous material upon a rotatable tapered driven roll having a porous surface while a gas at an elevated temperature is expelled outwardly through the surface of the porous roll and penetrates the fibrous configuration to the strand of product stabilization.

FIG. 2 is a cross section of the tapered porous roll shown in the apparatus of FIG. 1.

FIG. 3 is a perspective view of a portion of an alternative apparatus arrangement in which the strand undergoing treatment is continuously wound in a plurality of turns and continuously unwound from a rotatable porous roll having an essentially cylindrical configuration as well as upon a rotatable skewed roll in a paired spaced relationship to the porous roll.

DETAILED DESCRIPTION OF THE INVENTION The stabilized materials formed in accordance with the present invention retain the original fibrous configuration of the organic polymeric precursor, possess an enhanced thermal stability, andare capable of undergoing carbonization by heating in a non-oxidizing atmosphere according to procedures known in the art, e.g. heating in a nitrogen atmosphere at 1000 C. The stabilized strands formed in the present process may alternatively be used as heat resistant fibrous insulative materials, or in other applications where a thermally stable fibrous material is required. Additionally, the fibrous materials when derived from an acrylic precursor are non-burning when subjected to an ordinary match flame and find utility in the formation of fiame resistant fabrics.

The organic polymeric materials which are treated in accordance with the present invention may be of varied composition. Those precursors which are capable of undergoing thermal stabilization may be selected. For instance, the organic polymeric precursor may be an acrylic polymer, a cellulosic polymer, a polyamide, a polybenzimidazole, polyvinyl alcohol, etc. As discussed hereafter, acrylic polymeric materials are particularly suited for use in the present process. Illustrative examples of suitable cellulosic materials include the natural and regenerated forms of cellulose, e.g. rayon. Illustrative examples of polyamide precursors include the aromatic polyamides such as nylon 6T which is formed by the condensation of hexamethylenediamine and terephthalic acid. An illustrative example of a suitable polybenzimidazole is poly-2,2-m-phenylene- 5,5'-bibenzimidazole.

The acrylic polymer selected for use as the precursor may be formed primarily of recurring acrylonitrile units. For instance, the acrylic polymer should generally contain not less than about 85 mol percent of recurring acrylonitrile units with not more than about mol percent of a monovinyl compound which is copolymerizable with acrylonitrile such as styrene, methyl acrylate, methyl methacrylate, vinyl acetate, vinyl chloride, vinylidene chloride, vinyl pyridine and the like, or a plurality of such monomers.

In a preferred embodiment of the invention the starting material is selected in accordance with the teachings of U.S. Ser. No. 749,957, filed Aug. 5, 1968, of Dagobert E. Steutz, which is assigned to the same assignee as the instant invention, and is herein incorporated by reference. More specifically, the copolymer should contain no more than about 5 mol percent of one or more monovinyl comonomers copolymerized with acrylonitrile. In a particularly preferred embodiment of the invention the acrylic polymer is an acrylonitrile homopolymer.

The strand of polymeric fibrous material which is treated in accordance with the present invention is preferably a continuous multifilament yarn which may be formed by conventional techniques. Various threads, ropes, and cables, or continuous lengths of similar fibrous configurations may be selected. The strand which serves as the starting material may optionally be provided with a twist which tends to improve its handling characteristics. For instance, a twist of about 0.1 to 5 t.p.i., and preferably about 0.3 to 1.0 t.p.i. may be utilized.

The fibrous material which serves as the starting material optionally may be highly oriented. Such orientation is generally capable of enhancing the tensile properties of the resulting stabilized fibrous material, as well as of any carbon materials derived therefrom. An acrylic starting material may be oriented by hot drawing to a relatively high single filament tensile strength of at least about 4 grams per denier prior to stabilization. For instance, fibrous acrylic starting materials which possess a single filament tensile strength of about 4 to 9 grams per denier may be selected for use in the process.

As will be apparent to those skilled in the art, the atmosphere provided in the heat treatment zone may be varied. For instance, a cellulosic precursor is commonly stabilized in (1) an oxygen-containing atmosphere or in (2) an inert or non-oxidizing atmosphere, such as nitrogen, helium, argon, etc. Additionally, precursors such as an acrylic polymer, a polyamide, a polybenzimidazole, or polyvinyl alcohol are commonly stabilized in an oxygencontaining atmosphere. Air may be conveniently selected as the oxygen-containing atmosphere for use in the process. If desired, the precursor may be preliminarily treated with catalytic agents which are capable of promoting the stabilization reaction. When the stabilization treatment is conducted in an oxygen-containing atmosphere, it is commonly termed a preoxidation treatment particularly when followed by carbonization which is conducted in an inert atmosphere.

The stabilization zone is substantially enclosed in order to facilitate the confinement and withdrawal of off gases and/or the maintenance of an appropriate atmosphere. When a non-oxidizing'atmosphere is desired within the heat treatment chamber the strand may pass through a seal as it continuously enters and leaves the heat treatment chamber in order to exclude oxygen.

The stabilization of fibers of acrylonitrile homopolymers and copolymers in an oxygen-containing atmosphere involves (1) an oxidative cross-linking reaction of adjoining molecules as well as (2) a cyclization reaction of pendant nitrile groups to a condensed dihydropyridine structure. While the reaction mechanism is complex and not readily explainable it is believed that these two reactions occur concurrently, or are to some extent competing reactions.

The cyclization reaction involving pendant nitrile groups which occurs upon exposure of an acrylic fibrous ma terial to heat is generally highly exothermic and, if uncontrolled, results in the destruction of the fibrous configuration of the starting material. In some instances this exothermic reaction will occur with explosive violence and result in the fibrous material being consumed by flame. More commonly, however, the fibrous material will simply rupture, disintegrate and/or coalesce when the critical temperature is reached. As the quantity of comonomer present in an acrylonitrile copolymer is increased, a fibrous material consisting of the same tends to soften at a progressively lower temperature and the possible destruction of the original fibrous configuration through coalescence of adjoining fibers becomes a factor of increasing importance. The critical temperature referred to herein is defined as the temperature at which the fibrous configuration of a given sample of acrylic fibrous starting material will be destroyed in the absence of prior stabilization.

In a preferred embodiment of the invention the acrylic starting material exhibits a critical temperature of at least about 300 C., e.g. about 300 C. to 330 C. In addition to visual observation, the detection of the critical temperature of a given acrylic fibrous material may be aided by the use of thermoanalytical methods, such as differential scanning calorimeter techniques, whereby the location and magnitude of the exothermic reaction can be measured quantitatively.

The strand of polymeric fibrous material is continuously wound a plurality of turns in a single strand thickness and continuously unwound from at least one cantilevered rotating roll having a porous surface while a gas at an elevated temperature is expelled outwardly through the surface of the porous roll and penetrates the fibrous configuration of the strand wound upon the roll. The roll or rolls are preferably positioned within an essentially enclosed heat treatment chamber or zone whereby stabilization off gases may be confined and withdrawn and/ or the appropriate atmosphere maintained. As the fibrous strand travels a circuitous path around the periphery of one or more porous roll it is stabilized. The porous roll may be in the configuration of (1) a truncated cone [see FIGS. 1 and 2] or (2) it may be cylindrical having an essentially uniform diameter [see FIG. 3].

As illustrated in FIG. 1, when the porous roll has a conical or tapered configuration, the strand is passed onto the rotating roll surface at one end and is wound around the roll in a plurality of turns prior to leaving the porous roll surface at the other end. A strand of substantial length may be continuously stabilized within a relatively small area. The strand is wound about the periphery of the roll in such a manner that it is in intimate contact with the rotating porous surface of the roll and moves along the rotating roll in a helical or spiral path. A heated gas is continuously expelled through the surface of the porous roll which effectively penetrates the fibrous configuration of the strand. If desired, a plurality of porous conical or tapered rolls may be provided within the heat treatment zone. The rolls may be intergeared for actuation in unison according to known techniques. The strand may accordingly pass from the narrow end of one rotating roll to the wide end of an adjacent rotating roll where the stabilization is continued. If desired, the gas may be expelled from the surfaces of a series of rolls at successively elevated temperatures.

In another embodiment of the invention at least one cylindrical porous rotatable driven roll 1 is provided in a paired relationship to a rotatable skewed roll 2, i.e. an inclined roll, as illustrated in FIG. 3. Alternatively, tapered roll pairs may be utilized with the taper of each roll being in the same ratio within a given pair. The resulting pair of rolls may be positioned in the same or a different plane. The pair of rolls is aligned so that when rotated at fibrous strand wound or looped about the same will track from one end of the porous roll to the other while retaining a thickness of a single strand. Preferably the porous roll is of a substantially larger diameter than the skewed roll. The skewed roll optionally may be driven and provided with a porous surface through which a gas at an elevated temperature is also expelled. If not driven the skewed roll may idle, i.e. be rotated by the movement of the fibrous strand wound thereon. The rolls are preferably provided in a single plane and aligned so that their axes converage slightly. As will be apparent to those skilled in the fiber art, the strand will track the length of the heated roll and move towards the end where the axes converge. The lesser the degree of convergence the lesser 6 the rate of the lateral movement of the strand along the length of the heated roll.

The strand is passed to each roll at an angle essentially perpendicular thereto. When a skewed roll is utilized, the strand tends to leave each roll for passage to the roll paired therewith at an angle other than If desired, the strand may be introduced into the heat treatment zone for a plurality of passes in the event the size of the heat treatment chamber is inadequate to complete stabilization during a single pass.

The strand is treated while in contact with one or more rotating roll having a porous surface while a gas at an elevated temperature is expelled outwardly through the surface of the porous roll and penetrates the fibrous configuration of the strand wound upon the roll as described at least until the strand attains a stabilized form which retains its original fibrous configuration essentially intact. The resulting stabilized strand is commonly black in appearance. The treatment times during which the strand contacts a porous roll and temperatures will vary depending upon the composition and the exact configuration of the strand of polymeric fibrous material. Temperatures are selected which may be withstood by the strand without the destruction of its original fibrous configuration, e.g. through softening, melting, an uncontrolled exothermic reaction, or decomposition. The higher the temperature of the gas expelled through the porous roll generally the greater the rate at which the stabilization reaction occurs. When the fibrous strand is cellulosic in nature, e.g. rayon or cellulose acetate, the stabilization treatment may commonly be conducted at about 200 to 320 C. for a porous roll contact time of about 5 to 180 minutes. When the fibrous strand is a polyamide, e.g. nylon 6T, the stabilization treatment may commonly be conducted at about 200 to 350 C. for a porous roll contact time of about 5 to minutes. When the fibrous strand is a polybenzimidazole, e.g. poly-2,2'-m-phenylene-5,5-bibenzimidazole, the stabilization treatment may commonly be conducted at about 400 to 550 C. for a porous roll contact time of about 2 to 30 minutes. When the fibrous strand is polyvinyl alcohol, the stabilization treatment may commonly be conducted at about 180 to 200 C. for a porous roll contact time of several hours. More highly elevated temperatures may optionally be utilized during the final portion of the stabilization treatment.

When the strand of polymeric fibrous material is an acrylonitrile homopolymer or an acrylonitrile copolymer containing at least about 95 mol percent of one or more monovinyl units copolymerized therewith, the stabilization temperature is preferably selected in accordance with the teachings of commonly assigned U.S. Ser. No. 749,957 of Dagobert E. Stuetz which is herein incorporated by reference. With such starting materials the gas expelled through the surface of the porous roll may be provided at a temperature of about 260 C. up to about 10 C. below the critical temperature of the starting material. The entire stabilization treatment may be conducted by heating the fibrous material at a temperature substantially within the above range. In a preferred embodiment of the invention a strand of an acrylonitrile homopolymer fibrous material is heated at about 260 C. to about 300 C. and at a temperature of about 270 C. to 290 C. in a particularly preferred embodiment of the invention.

The stabilization treatment of the present process may be expedited if the gas is expelled through the surface of one or more porous roll over which the strand passes at successively elevated temperatures as the strand travels through the heat treatment chamber. For instance, the gas which penetrates the surface of a given porous rotating roll may be supplied to the roll surface at successively elevated temperatures along the length of the roll. Alternatively, a plurality of porous rolls may be provided in series with each successive roll expelling a gas through its surface at a more highly elevated temperature than the previous roll over which the strand passed. If desired, when stabilizing a strand of the preferred acrylic precursor the gas may be initially expelled at a temperature slightly below 260 C. and subsequently increased to at least about 260 C. where a substantial portion of the stabilization reaction occurs. During the final portion of the stabilization reaction the critical temperature exhibited by the unmodified starting material may commonly be exceeded. When it is desired to produce a strand of stabilized acrylic product which is capable of being carbonized or carbonized and graphitized to a product of high tensile strength, the present continuous process optionally may be conducted on a multiple stage basis in accordance with the teachings of US. Ser. No. 749,959 of Michael 1. Ram, filed Aug. 5, 1968 (now US. Pat. No. 3,539,295), which is assigned to the same assignee as the instant invention, and is herein incorporated by reference.

A strand of acrylic fibrous material is subjected to the stabilization treatment for a residence time sulficient to impart a bound oxygen content of at least about 7 percent by weight. Higher bound oxygen contents, e.g. up to 15 percent or more, may be achieved upon more extended treatment. The weight percentage of bound oxygen present in the material may be determined by routine analytical techniques, such as the Unterzaucher analysis. When operating with a strand of acrylonitrile homopolymer or an acrylonitrile copolymer containing at least about 95 mol percent of recurring acrylonitrile units and up to about mol percent of one or more monovinyl units copolymerized therewith at a stabilization temperature of at least about 260 C., stabilization residence times may commonly range from about minutes to about 150 minutes. More extended residence times (e.g. up to 24 hours) are commonly required when utilizing acrylonitrile copolymers containing a higher percentage of comonomer and stabilization temperatures substantially below 260 C., e.g. 200 C.

When utilizing a plurality of rotating porous rolls in the heat treatment zone, it is possible to vary slightly their relative speeds of rotation or the relative roll circumferences in order to accommodate shrinkage or stretching of the strand as it passes between the rolls should this be desired. When the strand is wound along the circumference of a single tapered porous roll, shrinkage or stretching of the strand optionally may be carried out depending upon whether the strand initially contacts the porous tapered roll at its wide end or its narrow end respectively, and whether the particular strand has a propensity to shrink or to stretch at the temperature of the gas being expelled.

The gas expelled through the surface of the porous roll when conducting the present process may be raised to the desired elevated temperature by any convenient means. In a preferred embodiment of the invention the gas passes over one or more electric resistance heating units positioned within the interior of each porous roll.

The strand of stabilized fibrous material produced in the present process may optionally next be carbonized and graphitized at more highly elevated temperatures in a non-oxidizing or inert atmosphere, according to tech niques known in the art. During the carbonization reaction elements present in the strand of stabilized material other than carbon, e.g. nitrogen, hydrogen and oxygen, are substantially expelled. The term carbonized as used herein is defined to describe a product consisting of at least about 90 percent carbon by weight. Suitable atmospheres include nitrogen, argon, helium, etc. A particularly preferred carbonization or carbonization and graphitization process is disclosed in commonly assigned U.S. Ser. No. 777,275, filed Nov. 28, 1968 of Charles M. Clarke (now abandoned) which is herein incorporated by reference.

The following examples are given as specific illustrations of the inventions. It should be understood, however,

EXAMPLE I An acrylonitrile homopolymer is dry spun to produce a 40 fil continuous yarn, and is hot drawn at a draw ratio of about 7.5 :1 to obtain a highly oriented fibrous material having a single filament tenacity of about 7 grams per denier. Twenty ends of this yarn are then plied to produce the 800 fil strand having a total denier of about 1150 which is utilized as the starting material.

As shown in FIG. 1, the strand of acrylic fibrous material may be provided on a bobbin 4 located outside the substantially enclosed heat treatment chamber 6 having walls 8. Situated within the heat treatment chamber 6 is provided a cantilevered rotatable roll 10 having the configuration of a truncated cone and a porous surface. The Wide end 12 of the roll has a diameter 5 inches, and the narrow end 14 of the roll has a diameter of 4.5 inches. The length of the roll 18 is 24 inches. The rotatable roll 10 is mounted upon a hollow conductive shaft 16 which extends through a seal in the wall of the heat treatment chamber 6, and is driven by the aid of pinion 18 and worm gear 20. The hollow conductive shaft is bolted to supporting member 21 with the aid of fixed end plate 22 and is rendered rotatable by hearing 24.

Air is continuously supplied at an adjustable overpressure to the end 28 of the hollow conductive shaft 16 which extends through supporting member 21 by a fan (not shown). A multi-lead cable 30 extends into hollow shaft 16 and supplies electrical current to resistance heaters (not shown) situated within rotatable roll 10. Air introduced into hollow shaft 16 at end 28 passes over resistance heaters and is continuously expelled outwardly through the porous surface 32 of rotatable roll 10.

As shown in FIG. 2, the hollow conductive shaft of conduit 16 is coextensive with the length of rotable roll 18 and further extends through seal 34 in the opposite wall of the heat treatment chamber 6 where it is capped. Such extension of hollow conductive shaft 16 through the wall of the chamber lends additional support to the rotatable roll 10. Non-conductive end plates 36 and 38 are situated at the wide and narrow ends respectively of rotatable roll 10. Additional dividers 40, 42, 44, and 46 of a non-conductive material are positioned around the circumference of central conduit 16 and define a series or" heating modules 48, 50, 52, 54, and 56 within the interior of rotatable roll 10. The porous surface 32 of roll 10 is formed of a sintered ceramic material through which air may be readily expelled outwardly. Situated within heating modules 48, 50, 52, 54, and 56 are conductive screens 58, 60, 62, 64, and 66 which are welded to pairs of conductive rings 68, 7t), 72, 74, and 76. One member of each pair of conductive rings is welded to hollow conductive shaft 16.

Multi-lead cable 30 includes a series of five individual leads 78, 80, 82, 84, and 86 which pass through oversized openings in hollow conductive shaft 16 lined with insulative bushings 88, 90, 92, 94, and 96. The individual power leads 78, 80, 82, 84, and 86 are welded to one member of each pair of conductive rings 68, 70, 72, 74, and 76 respectively. Electric curent is supplied to each individual power lead 78, 80, 82, 84, and 86 from a variable power source 98 with a sliding connector or wiper arrangement 100 of the concentric ring type.

As the strand of acrylic fibrous material is unwound from bobbin 4 at a rate of 2 metres/minute, it passes through an opening in the wall 8 of the heat treatment chamber 6 and meets the porous surface 32 of rotating roll 10 at its wide end 12. The strand winds 11 turns per inch along the axial length of rotating roll 10 in the absence of strand overlap. Electric current is supplied to individual leads 78, 80, 82, 84, and 86 so that conductive screens 56, 68, 62, 64, and 65 impart a uniform temperature of 270 C. to the air which is expelled outwardly through the porous surface 32 of roll 10. Such air penetrates the fibrous configuration of the strand wound upon roll 10 and stabilizes the same. The contact time of the strand upon roll 10 is 50 minutes. As shown in FIG. 1 the resulting stabilized strand is continuously unwound from the narrow end 14 of rotating roll 10 and withdrawn from heat treatment chamber 6 through opening 102 and wound upon bobbin 104.

Off gases from the stabilization treatment are removed from heat treatment chamber 10 through exhaust duct 106, with the aid of fan 108. If desired, gases removed from duct 106 may be scrubbed and at least partially recycled to the end 28 of hollow conductive shaft 16.

The resulting stabilized strand present upon bobbin 104 is black in appearance, exhibits a bound oxygen content of 8 percent by weight as determined by the Unterzaucher analysis, retains its original fibrous configuration essentially intact, and is non-burning when subjected to an ordinary match flame.

The stabilized strand may be woven to form a fire resistant fabric, or carbonized or carbonized and graphitized in an inert atmosphere in accordance with the teachings of US. Ser. No. 777,275, filed Nov. 20, 1968 (now abandoned) which is herein incorporated by reference. The carbonized or carbonized and graphitized fibrous products find particular utility as a reinforcing medium when suspended in a suitable matrix material to form an article useful as a strong lightweight structural component.

EXAMPLE 11 Example I is repeated with the exception that the air expelled outwardly through the porous surface 32 of roll 10 at heating modules 48, 50, 52, 54, and 56 is at 260 0, 275 0., 310 0., 350 C., and 400 C. respectively. The fibrous strand penetrated by the heated air is passed continuously over each module for a residence time of approximately miuutues. Substantially similar results are achieved.

EXAMPLE H1 The stabilization procedure of Example I is repeated with the exception that the strand of starting material is a 740 fil rayon continuous filament yarn having a total denier of about 2200, the air expelled outwardly through the surface 32 of roll is maintained at a uniform temperature of 220 C., and the strand is penetrated by the heated air while passing along the roll for a residence time of 60 minutes.

The resulting strand retains its original fibrous configuration essentially intact, exhibits enhanced thermal stability, and is capable of undergoing carbonization.

EXAMPLE IV The stabilization procedure of Example I is repeated with the exception that the strand of starting material is a 300 fil continuous filament yarn of nylon 6T (condensation product of hexamethylenediamine and terephthalic acid) having total denier of about 900, the air expelled outwardly through the surface 32 of roll 10 is maintained at a uniform temperature of 315 C., and the strand is penetrated by the heated air while passing along the roll for a residence time of 90 minutes.

The resulting stabilized strand retains its original fibrous configuration essentially intact, exhibits enhanced thermal stability, and is capable of undergoing carbonization.

EXAMPLE V The stabilization procedure of Example I is repeated with the exception that the strand of starting material is a 400 fil continuous filament yarn of poly-2,2'-m-5,5'-bibenzirnidazole (a preparation of which is described in Ex- 10 ample H of US. Pat. No. 3,174,947) having a total denier of about 1600, the air expelled outwardly through the surface 32 of roll 10 is maintained at a uniform temperature of 470 C., and the strand is penetrated by the heated air while passing along the roll for a residence time of 5 minutes.

The resulting stabilized strand is non-burning when subjected to an ordinary match flame and retains its original fibrous configuration essentially intact.

Although the invention has been described with preferred embodiments, it is to be understood that wariations and modifications may be resorted to as will be apparent to those skilled in the art. Such variations and modifications are to be considered within the purview and scope of the claims appended hereto.

I claim:

1. A process for the thermal stabilization of a strand of a fibrous acrylic material selected from the group consisting of an acrylonitrile homopolymer and an acrylonitrile copolymer containing at least about mol percent of acrylonitrile units and up to about 5 mol percent of one or more monovinyl units copolymerized therewith comprising passing said strand through a substantially enclosed heat treatment zone wherein said strand is continuously wound in a plurality of turns and continuously unwound from at least one rotating roll having a porous surface for a residence time of about 10 to about minutes while in contact with said porous surface while an oxygen-containing gas at a temperature of at least about 260 C. is expelled outwardly through the surface of said porous roll and penetrates the fibrous configuration of said strand wound upon said roll at successively elevated temperatures with said gas being preheated to said successively elevated temperatures by electric resistance heating means situated within the interior of said porous roll or porous rolls prior to being expelled outwardly through said surface thereof, and continuously withdrawing the resulting stabilized strand of fibrous material from said substantially enclosed heat treatment zone which is black in appearance, non-burning when subjected to an ordinary match flame, has a bound oxygen content of at least 7 percent by weight, and retains its original fibrous configuration essentially intact.

2. A process according to Claim 1 wherein said fibrous acrylic strand is an acrylonitrile homopolymer.

3. A process according to Claim 1 wherein said oxygen-containing gas is air.

4. A process according to Claim 1 wherein said strand of acrylic fibrous material while wound upon at least one porous rotating roll is penetrated by a gas which is supplied at successively elevated temperatures within the range of about 260 to 300 C.

5. A process according to Claim 1 wherein said rotating roll having a porous surface has the configuration of a truncated cone.

References Cited UNITED STATES PATENTS 3,567,380 3/ 1971 Townsend 423-447 3,512,265 5/1970 Fleissner 68--DIG. 5 2,952,033 9/ 1960 Goodwin 8-130.1 X 2,455,173 11/1948 Hitt 8-130.l X 2,002,996 5/1935 Hoefinghoif et a1. 8-151.1 3,539,295 11/ 1970 Ram 423-447 3,027,222 3/ 1962 Wilkinson 8--115.5

FOREIGN PATENTS 1,502,253 10/ 1967 France 423-447 1,170,591 1 1/ 1969 Great Britain 423-447 DONALD LEVY, Primary Examiner US. Cl. X.R. 

